SUMMARY/ABSTRACT Still needed are immunotherapies sufficiently robust to suppress ? cell autoimmunity and safely prevent and treat type 1 diabetes (T1D) in the clinic. This need is becoming more urgent in the face of an increasing incidence of T1D in the United States and world-wide. We previously reported that a short-course of nondepleting (ND) antibodies (Ab) specific for the T cell coreceptors CD4 and CD8? reverses diabetes in the majority of new onset NOD mice, and that remission is indefinite. Self-tolerance is reestablished in a tissue- specific manner and acquired immunity is unaffected. Notably, recent findings demonstrate that ND Ab specific for human CD4 and CD8? engineered by our group exhibit similar tolerogenic properties in humanized mouse models. Ongoing work has made the exciting observation that coreceptor therapy impacts both central and peripheral tolerance, involving a number of novel mechanisms. Accordingly, the goal of this proposal is to define the molecular and cellular events regulated by coreceptor therapy in the thymus and periphery that drive long-term tissue-specific tolerance. In AIM 1, we will investigate the mechanisms by which coreceptor therapy regulates the efficiency of thymic selection. In AIM 2, work will focus on determining how coreceptor therapy influences T cell-mediated peripheral immunoregulation, and the pathogenicity of anti-self T cells. In both Aims, experiments will exploit our ND anti-human CD4 and CD8? Ab to test the in vivo effects of coreceptor therapy on thymocyte development and peripheral tolerance in humanized mice. Insight gained via the proposed studies will establish treatment parameters for successful and safe clinical application of the approach. Importantly, coreceptor therapy will be applicable not only for the prevention and treatment of T1D, but also for: i) other T cell-mediated autoimmune diseases and pathologies, as well as ii) induction of transplantation tolerance.